This invention relates generally to devices for testing the tension of tennis rackets, and more particularly to improvements therein affording improved accuracy and providing for the accommodation of rackets of different manufacture and configuration.
In the above-mentioned copending application, there is disclosed a testing device having a generally cylindrical housing which engages one side of the string grid and a thumb wheel which engages segments of the engaged strings on an opposite side thereof, and which displaces the central area of the strings a given distance to create a measurable strain which is, in turn, determined by a known electronic strain gauge mounted upon or associated with a deflectable beam, the strain gauge providing data which is processed to provide a digital readout.
While this structure represents a substantial improvement over prior art devices, we have found that where the device is used for measuring string tension in a variety of racket styles having strings of varying diameter and string spacing, the degree of accuracy is not constant, and in some cases not capable of repetition. This problem appears to be due to the fact that the degree of deflection required is quite small, and in some cases, is approximately equal to the diameter of the string. If the means used to deflect the strings operate in a manner that the diameter of the string determines the degree of deflection, as, for example, where the strings are moved against a surface which determines the degree of deflection, the actual lateral displacement of the string grid will vary accordingly, and a supposed standard displacement will not, in fact, occur. Thus, similar rackets, strung to an identical tension, using strings of different diameters will not be deflected to the same degree, and will provide different tension readings.
In the device described in the above-mentioned application, the generally centrally disposed strings forming the racket face are deflected within an enclosed area determined by a generally circular housing. While in theory, this should provide a more accurate reading, in fact, we have found that the use of the housing in which only the enclosed string segments participate in deflection tend to provide a displacement-string relation which is other than substantially linear within the range of interest, thus tending to result in readings which are less accurate than desirable.
Further, with the use of an electronic strain gauge to provide tension readings, no provision has been made in the prior art constructions to provide a "zero point" which will cancel out commercial manufacturing tolerances, variations in string diameter, and other inaccuracies inherent in testing devices of the described type.